1. Field of the Invention
The invention relates to a metal sub-frame for a motor vehicle and to a method for producing such a sub-frame.
2. Description of the Related Art
What is intended by "motor vehicle" is any motorized four-wheeled vehicle, particularly utility vehicles and touring cars.
In this description and in this patent, what is meant by "sub-frame" is a structural component, generally made of metal, for a motor vehicle, providing a direct or indirect connection between, on the one hand, the body of the vehicle and, on the other hand, either the front wheels or the rear axle of said vehicle. A sub-frame of this kind has a central region consisting of a profile which needs to have sufficient transverse stiffness that it can withstand the bending stresses created by the movement of the wheels, to each end of which central region there are secured two profiled arms which, directly or indirectly and on each side of this central region, provide a connection with the body and with a front wheel wishbone or the rear axle.
What is meant by "front wheel wishbone" in this patent is the component conventionally known as the wishbone or any part which has a function equivalent to that of a wishbone.
The front sub-frame of a motor vehicle thus generally has, at each of the two ends of the central region, means of connection to the body and means of connection to a front wheel wishbone. It also generally has attachment means for a torque rod connected to the engine, for an anti-sway bar and for the steering rack of the vehicle. A front sub-frame of this kind therefore needs to be strong enough that it can withstand the bending forces generated by the movement of the wheels, and take up the engine torque and the forces coming from the anti-sway bar and the steering rack. Another function of this sub-frame is to damp and filter all of these forces and vibration and noise resulting from them, for the comfort of the passengers of the vehicle.
The rear sub-frame of a motor vehicle generally has, at each of the two ends of the central region, means of connection to the body and means of connection to the rear axle assembly. Its principal function is to provide a second stage of filtering of the horizontal and vertical forces and of vibration and noise resulting therefrom, the first stage of filtering being performed by the rear axle assembly itself, the purpose of this being further to improve the comfort of the passengers; this solution employing a rear sub-frame is quite expensive and is chiefly used in so-called top-of-the-range vehicles. This rear sub-frame also needs to be mechanically able to withstand the forces to be filtered, although these forces impose far less stress than in the case of the front sub-frame.
Metal sub-frames of this kind are already known and consist of two pressed half shells welded together longitudinally. Each pressed half shell has an open profiled central region with the transverse shape of a U, which is extended at each of its two ends by two open profiled arms in the shape of a U, the assembly having the longitudinal shape of an H. A longitudinal weld, which may or may not be continuous, connects the edges of the open U-profiles of the central region and of the arms of each half shell. The two closed profiled arms at each end of the central region, which consist of open profiled arms joined together by the weld, are used for securing the body, generally by two connecting points, and the other for securing, at one or more connecting points, either the wishbone of a front wheel or the rear axle. The securing of other elements may be envisaged, for example, in the case of the front sub-frame, these might be elements such as the steering rack, the engine torque rod and an anti-sway bar, these being secured via attachment means.
A sub-frame of this kind displays a number of drawbacks. It is expensive to manufacture because it requires two pressing operations starting from shaped metal blanks, and this causes material to be wasted. For making the longitudinal weld it is also necessary to provide a strip of metal along each half shell, which strip has no other use, and this makes the sub-frame heavier and increases its size. What is more, the weld constitutes a region which is more vulnerable to the repeat stresses applied to the sub-frame, particularly the bending stresses generated by the movement of the wheels. Finally, the sub-frame thus produced is a closed piece, and this makes it difficult for the previously-described connecting and attachment means to be secured in a convenient way that can withstand the stresses.
The invention has sought to overcome these drawbacks and to design a metal sub-frame which, while retaining enough rigidity that it can take up the forces applied to it, allows the wasting of metal in manufacturing it to be minimized, its weight to be reduced, which is very favorable in the case of a vehicle, its compactness to be increased, which makes the overall design of the vehicle easier, the welded regions which are vulnerable to repeat stresses to be minimized, and finally, allows more robust securing of the connecting and attachment means.